We have previously demonstrated that antisera directed against specific immunoglobulin (Ig) heavy chains can be used to accomplish both complete and class-specific humoral immunosuppression in mice. We now propose to explore the medical significance of this observation in three general areas of study. 1. We have very recently reported that congenitally athymic (nude) mice retain healthy human skin grafts for the lifetime of the recipients but reject chicken skin in under 20 days. Class-specific anti-Ig immunosuppression will be applied to this system in order to elucidate the role of humoral antibodies in the transplantation rejection mechanism. 2. Anti-Ig immunosuppression will be used to study two types of malignant processes. Direct suppression of virally induced murine leukemia will be attempted by subjecting Ig- bearing leukemic cells, or their precursors, to direct attack by anti-Ig antisera. An attempt will also be made to establish a general model for the inhibition of neoplasms by suppressing formation of humoral "blocking antibodies." 3. Class-specific anti-Ig immunosuppression will be used to examine the involvement of each antibody class in a number of selected bacterial disease processes.